CAMBRIDGE has entered a period of significant change, the City's six square miles are crammed with an incredible variety of industries, businesses, homes, and institutions, and the possibility of altering things would seem, at first, minimal. It isn't.

The City already faces an accelerating series of physical changes, some of which have already started, and others which should begin within the next decade:

* The National Aeronautics and Space Administration is building a $60 million research laboratory near M.I.T. Next to the 29-acre NASA development, the city is planning a private urban renewal project of high rise buildings expected to accommodate many of the research and engineering firms attracted to Cambridge by NASA and M.I.T. The City, with the cooperation of M.I.T., has already sponsored one such project. Technology Square.

* The eight-lane Inner Belt high-way, which is still being fought by Cambridge politicians, will probably cut through the City several blocks east of Central Square, displacing between 3000 and 5000 residents.

* The John F. Kennedy Memorial Library will be built on the last large piece of undeveloped real estate in Harvard Square, the MBTA's Bennett St. repair yards, which will be relocated. Once completed, the Library is expected to attract hundreds of thousands of tourists a year. And the City administration is now talking seriously about plans for redeveloping other parts of Harvard Square.

* The city dump, long an eyesore and inconvenience for the surrounding neighborhood which has had to endure blowing garbage and billows of smoke, will be replaced by an incinerator. The real dividend for Cambridge will be new land for development; the dump is one of the last remaining large tracks of virgin real estate in the City open for public or private use.

* The MBTA will extend its Harvard Square line into North Cambridge, with one stop at Porter Square and another at Alewife Brook Parkway, where new repair yards will be located to replace the Bennett St. facilities across from Eliot House.

These changes are coming, but no one predicts confidently what--if any--the long range consequences for the City will be. In fact, no one is even sure of the precise shape the changes will take. The Inner Belt, for example, has been in discussion for nearly two decades. The relocation problem it will create is staggering; many think it is insoluble. Yet because it was politically dangerous to imply an acceptance that the high-way was coming, a start on relocation is only now being made.

The city dump, despite its recognized potential for massive redevelopment, is another area where specific plans are nowhere in sight. The Planning Board, the Historic Commission, and other like-minded groups believe that the area should be planned in a systematic, coordinated fashion. Other interests--primarily land-hungry businesses, so far--seem to be pressing for a quicker and less lofty disposition of the land. It is obvious that the dump will someday will not be a dump, but the nature, speed, and desirability of the different alternatives are obscure. Likewise the redevelopment of Harvard Square, much discussed in general terms, has so far escaped details.

But Cambridge is not only facing a period of major and uncertain physical change. It is also experiencing less obvious, but no less important, social changes. The most prominent, most publicly-discussed of these is the influx of "transients."

The conventional way of seeing this trend is as a housing problem. There is little double that Cambridge has become a magnet for large numbers of students, young single workers, and young professionals. They are placing a tremendous strain on the local housing supply. They have flooded areas to the north and east of Harvard Square, and they are turning up in increasing numbers around Gentral Square and even in East and North Cambridge. In the process, many buildings in the City have been converted and rehabilitated, rents have gone up, and--according to the commonly-accepted theory--Cambridge residents have been forced out of the City to look for housing elsewhere.

The result is the housing "shortage." There is little evidence to contradict the existence of a real squeeze, and most City officials who watch the housing situation believe there will be a continuing demand for more space among young people who want to live in Cambridge. The prospect, then, is for more of the same: more transients, low rents getting higher, and low-income Cambridge residents being forced out of the City. The next logical area for these "market forces" to work seems to be eastern, most residential areas of the City.

To the existing pressures are added the probability of the Inner Belt's taking more than 1200 housing units and the possibility of business or commercial expansion around NASA and M.I.T. It was the combination of these forces that led the planners to propose a model cities project for 268 acres in East Cambridge. The City's proposal perceived the problem this way:

Too often, there is a tendency to view the low-income neighborhood as expendable... One chief thrust of the Cambridge proposal is to confront that issue directly, and to test many techniques for preserving a low-income area for its residents and for injecting new and valuable resources into its way of life.

There is no guarantee that Cambridge will have its model cities application approved, and even if it does, there is less certainty that the techniques proposed in the application will work. Though there is also no certainty in the predictions that the low-income residents in Eastern Cambridge will become increasing beleagured (and many of them slowly replaced), the hypothesis has become the Conventional Wisdom.